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I I I I I I I I I I I I I I I I I I I 54 A final point to consider is vegetation. Vegetation stabilizes bars and banks, and makes them much more resistant to erosion. This study did not directly assess the effects of vegetation on substrate mobility, but it was clear that many bars in the study area have become vegetated by willow and tamarisk. Since it is assumed that maintaining channel complexity is important to endangered fish habitats, attention should be given to suppressing vegetation since it inhibits the movement of the material making up bars and islands. It is likely that consecutive low flow years allow vegetation to become firmly established on channel bars, and thus make the gravels more difficult to entrain than our predictions show. Observations of the 1993 flow showed that many small plants survived this relatively large flow event, so either larger flows or consecutive years of high flow should be needed to remove vegetation from bars and islands in the reach. DOWNSTREAM HYDRAULIC GEOMETRY If flows are prescribed for maintenance and creation of fish habitats in the Grand Valley and Ruby-Horsethief Canyon, it is important to understand how these flows affect sediment transport for the entire reach. In order to do this, we measured main channel cross-sections to examine longitudinal trends in bankfull width, depth and shear stress. These measurements are essential for putting our flow modeling sites in a larger context, and for understanding the general character of the river. The information on the occurrence of wide and relatively shallow areas, which represent areas of complex channel, indicate the general frequency of this habitat type. Also, the calculation of bankfull dimensionless shear stress provides a measure of where bank erosion, and an increase in channel complexity, might occur. This can be estimated since bank erosion generally